Roots in the Topsoil at Establishment and Elongation Determine Seed Yield Response to Phosphorus in Select Brassica napus Cultivars

Oilseed rape (Brassica napus L.) is the main oil crops in China as well as in the world. In this study, we aim to incorporate key root traits into future Brassica napus cultivars to increase access to or uptake of phosphorus (P) from the soil during the entire growth stage. Four Brassica napus cultivars with contrasting root traits were selected to study the contribution of root morphological traits (total root length, TRL; coarse root length, CRL; fine root length, FRL; root surface area, RSA) and root physiological traits (acid phosphatase activity, APA; rhizosphere pH, Rhizos-pH; organic acid content, OAC) each growth stage on the shoot dry weight (SDW), seed yield (SY) and total P accumulation (TPA) under low (LP, 30 kg ha^-1) and high (HP, 90 kg ha^-1) P supplies in the field conditions. SY significantly correlated with SDW at HP in silique and ripening stages and at LP in each growth stage. TPA had a significant correlation with SDW in each growth stage at both P conditions. SY, SDW and TPA was strongly correlated with root traits, especially CRL1 and RSA1 in the topsoil (0-10 cm), at bolting and silique stages, while not correlated with APA, OAC and rhizos-pH at both HP and LP. Cultivar SWU93 had a relative higher SDW and RDW from bolting stage to ripening stage and higher seed yield than other cultivars under both HP and LP supplies. Factor analysis showed that cultivar SWU93 had a higher root morphological scores than root physiological scores in bolting and silique stages. These suggests that root morphological traits will be better predictors of SDW and SY than root physiological traits in Brassica napus in field conditions, and CRL and RSA in later growth stage will determine the SDW and SY in low P soils.